1. Field of the Invention
This invention relates to real time monitoring of radiation machinery, such as electron beam processors for industrial use.
2. Description of the Prior Art
X-ray monitors
There are numerous techniques used to monitor radiation producing equipment, largely developed for the accurate measurement of the radiation fields generated by x-ray equipment for medical applications. These range from mosaic detectors as used in computer assisted tomographic (CAT) scanners to large screen displays for radiation therapy planning and monitoring. All of these tools depend upon the detection of penetrating (10-1,000 KeV) photons in air, but none have been available for the direct diagnosis of the electron beam itself which generates the x-ray or photon distribution through the bombardment in vacuum of high Z metal targets, and through treatment in gas, at atmospheric pressure, of the product itself.
Direct Electron Processors
Over the past two decades, a large number of direct electron beam processors have been developed for industrial use. These range from the "spot beam" high voltage accelerators, such as the Dynamitron.RTM. type (manufactured by Radiation Dynamics Inc., Melville, N.Y.) to the large area, "distributed beam" low voltage accelerators, for example of the Electrocurtain.RTM. type (manufactured by Energy Sciences Inc., Wilmington, Mass.).
Electron window
Unlike an x-ray generator, these processors utilize an accelerator vacuum housing in which the accelerated electrons can be delivered directly to product located outside the vacuum housing in a controlled environment usually at standard temperature and pressure (STP). This is normally accomplished by replacing the x-ray producing heavy metal target (i.e. the anode) with an electron permeable window. These windows are typically made up of thin metallic foils (e.g. Titanium) offering high strength at elevated temperatures and sufficiently thin that modest (e.g. &lt;20%) electron energy loss occurs in transmission.
Window support structure
Since the power densities of the electron beams from these processors are very high at the window or anode planes (150 w/cm.sup.2), the windows must incorporate suitable heat dissipation techniques--usually conduction cooling of the metallic foil via support frames with water channels, or forced air convection cooling of the window foils. For electron processors below 300 KV, where cooling is particularly demanding, water cooling is mandatory for large area equipment. It is always employed in industrial units, either in the form of finned or drilled plates or honeycomb support frames with peripheral water cooling. The design of these window structures must optimize electron transmission because any electrons stopped or lost in the window represent a "full energy" loss; that is, the electrons at this (anode) plane have a full energy investment from the processor. The goal is to effect the maximum transfer of the kinetic energy carried by the accelerated beam into absorbed energy or dose in the target or product located on the exterior side of the window. Nevertheless, for high power processors, the energy losses in the window can be very large--typically ranging from 20% in the foil and 20% in the support frame for low energy processors (e.g. 150 KV), to 1.5% in the foil and nothing in the frame for scanned high energy processors (e.g. 2000 KV).
Thin film dosimeters
Most of the standard dosimetric techniques which have been developed (such as those described at McLaughlin, W. L., Humphreys, J. C., Hocken, D. and Chappas, W. I, "Radiochromic Dosimetry for Validation and Commissioning of Industrial Radiation Processes", Radiat. Phys. Chem. 31, #4-6, 505, (1988)) for penetrating radiation (x-rays and gamma-rays) cannot be applied in the industrial application of this machinery, due to the modest penetration capabilities of the electron energies used in practical process applications (0.1-2.0 MeV). Consequently, film dosimeters have been developed with thicknesses far less than the depth of penetration of a 100 KeV electron (typically 10-50 g/cm.sup.2). This dosimeter thickness feature is essential for accurate diagnosis of the electron spectra delivered by the machines because it is desirable to have dosimeter thicknesses some 5-10% of the range of the electrons being diagnosed, particularly for spectral evaluation. In this application, one uses depth-dose laminates to determine the effective penetration profile of the spectra in matter. This technique is widely employed for spectral quality verification in operating processors, and although laborious in practice, can provide spectral energy determinations accurate to within a few percent (e.g. .+-.1%).
Parameters requiring monitoring
In the application of electron beam machinery for industrial processing (as described for example at Nablo, S. V., "Electron Beam Processing Machinery", Ch.9, Radiation Curing in Polymer Science and Technology, ed. J. P. Fouassier and J. F. Rabek, Elsevier Applied Science, London (1993)), it is necessary to monitor the three critical operating parameters: machine yield, uniformity, and energy.
Yield
The first of these, yield k has units of Mrad*meters per min.*ma for a given machine, and relates machine output current I, and product speed v, to delivered dose D; i.e. D=kI/v. This yield parameter should be invariant with current if the electron optics of the system are well designed, but it will vary with electron energy or accelerator voltage because the electron stopping power in the product or in the dosimeter varies with energy. A typical yield variation with accelerator voltage (electron energy incident on the window) is shown in FIG. 8. The lower energy ends of these curves are dominated by window foil absorption while the fall-off at higher energies arises from the decrease in electron stopping power with increasing energy in all materials.
Sampling rate
The yield sampling rate will vary with the tolerance of the product for dose variation. For example, in electron sterilization of medical devices, yield data will be recorded and evaluated several times daily, before the process can be continued. This is mandatory even with continuous recording of other process parameters such as voltage, current, and conveyor speed.
Current measurement
One of the primary reasons for the development of the device disclosed here is the need to continuously monitor the electron beam reaching the window in order to verify the current indication given by the power supply itself. The indeterminate nature of the power supply return current as a measure of the actual beam current is depicted schematically in FIG. 9. Here the presence of gas molecules in the beam occupied region leads to a backstreaming ion current to the gun or high voltage terminal of the accelerator. The arrival of these positively charged ions is recorded by the ammeter I in the high voltage power supply circuit as electrons leaving, so their presence leads to errors in the measurement of the accelerated electron current (as described in more detail at Nablo, S. V., "Progress Toward Practical Electron Beam Sterilization", pp. 210-221 Sterilization of Medical Products, ed. E. R. L. Gaughran and R. F. Morrissey, Multiscience Publications Ltd., Montreal, Quebec (1981)). While these errors are insignificant at normal residual gas pressures in the accelerator (e.g. 10.sup.-6 mm Hg.) they can become large with a localized concentration of N.sub.2 and O.sub.2 introduced by pin-holes or leaks in the electron permeable window foil. The deterioration of a window foil on a 2 meter industrial processor foil as it approached failure is documented in FIG. 10 and this phenomenon has been reproduced under controlled conditions via gas injection in a working processor at the window plane. Furthermore, this problem and its consequences are exacerbated by the "trapping" of ions in the electron beam occupied region of the processor, so that significant gas density can build up due to pin-hole effects without affecting the ionization gauge or gas analyzer monitoring the residual gas pressure in the vacuum envelope of the accelerator.
Magnetic fields
An additional operational problem which can affect the performance of the processor is that due to the presence of time varying external magnetic fields which can distort the electron optics. The electron "rigidity" in these machines can vary greatly from, for example, an H.rho. value of 34 gauss-cm for 100 eV electrons in the triode region of the gun, to an H.rho. value of 2120 gauss-cm for 300 KeV electrons fully accelerated at the window plane. Stray fields from dc motors, overhead cranes or from the equipment on which the processor is mounted, can easily affect the electromagnetically unshielded low energy beam so that significant electron optical deterioration occurs; i.e. fewer of the accelerated electrons reach the window and are absorbed in the chamber wall instead. A demonstration of this is shown in FIG. 11, recorded with a 30 cm sterilizer in which the original uniformity profile is plotted, and then the profile with a permanent magnet generating a 3 gauss field orthogonal to the beam in the accelerator gap. As shown, a significant depression of the beam yield on the centerline resulted (-15%) while the Lorentz forces (j.times.B) on the beam led to a pile-up of the current density at the ends of the window. Tests have shown that very large yield errors (50%) can be introduced into these processors with relatively modest field strengths of appropriate orientation; e.g. 5 gauss fields perpendicular to the beam's direction of motion. This sensitivity is the result of the magnetically unshielded optics of these processors because of the non-ferromagnetic stainless steels used in their fabrication.
Invasive dosimetry
A final operational problem in the monitoring of electron processing is the "invasive" nature of dosimetry. One wishes to monitor the processor performance (as described for example at Nablo, S. V. and Frutiger, W. A., "Techniques for the Diagnosis of Industrial Electron Processor Performance", Rad. Phys. Chem 18, #5-6, 1023 (1981)) under actual production conditions. These may be at high processor power levels and, typically, high line speeds, (say 200 meters/minute). It is impractical to perform dosimetry "on the fly" at these speeds, so that dosimetric mapping must be performed by interruption of the product flow and slowing of the line speed. For example, in the curing of inks on paperboard, a dosimetric array is carried through the "non-printing system", usually at reduced line speeds and hence operational current levels.
Dosimeter range
In addition, there are high dose applications in which doses are given which are well outside the linear range of the film dosimeters; i.e. above 5 to 50 kGys. Here, measurements must be made at other than actual production conditions, and simple current or speed scaling used to confirm the actual production dose. For sterilization applications the dosimeter absorption problems are even more severe, especially where controlled depth of penetration is important. The additional thicknesses of the dosimeter and its carrier are intolerable because they reduce the dose delivered to the product, and the dosimeters must be mounted on an exemplar which can be handled by the conveyor in a manner identical to that in which the actual product is transported.
Need for Real Time monitor
All of these considerations have stimulated the search for a simple, non-invasive technique which permits processor monitoring in real time. It is necessary that the technique be traceable to a national standard. For example, in the United States the thin film dosimeters are traceable to (calibrated by) a standard Cobalt 60 source at the National Institute of Standards and Technology in Gaithersburg, Md., and any real time radiation monitor used for these machines must be traceable to similar national standards worldwide.
Cross web Uniformity and Energy
Many of the "yield" measurement problems reviewed above are common to the measurement of both cross web uniformity of the beam and of beam energy. Both determinations require invasive dosimetry, and the difficulties of handling large dosimeter arrays at elevated speeds for the cross-web measurement of uniformity requires skilled technique and prolonged interruption of production. This labor intensive procedure is rarely performed unless required for "regulated application" such as sterilization, in spite of the need of most processes for good edge-to-edge treatment uniformity. The same is true of the dosimetric depth-dose technique used for electron energy determination, in spite of the great sensitivity of many critical sterilization or polymerization processes to small changes in electron energy; i.e. depth of penetration.
Machine voltage measurement
Dependence upon machine voltage monitoring is unreliable over the long term. The major error mechanism here is the change in resistance in the high voltage resistive divider string. This is usually made up of a large number of precision high voltage resistors of the deposited film type; e.g. 10.sup.7 -10.sup.8 ohms. Such resistors are subject to aging and spark damage. In summary then, the quality assurance of all in-line radiation processing would benefit from a real time monitor which is free of the problems just enumerated.
References
The following publications indicate the background of the invention and illustrate the state of the art; they are hereby incorporated herein by this reference thereto.
References with respect to solid state radiation detectors include Knoll, G. F., Radiation Detection and Measurement, John Wiley & Sons, Inc., 2nd Edition (1989), Chapter 11, Semiconductor Diode Detectors, pp. 337-386.
References with respect to speed measurement using generating tachometers include Pallas-Areny, R. and Webster, J. G., Sensors and Signal Conditioning, John Wiley & Sons, Inc. (1991), 4.3.1.1 Generating tachometers, pp. 182.
References with respect to the use of look-up tables for calibrating instruments, as an alternative to formulae, in microprocessor based instruments include Pallas-Areny, R. and Webster, J. G., Sensors and Signal Conditioning, John Wiley & Sons, Inc. (1991), 1.4.2 Other Characteristics: Linearity and Resolution, pp. 13, 14.
References with respect to x-ray kV measurement using filtered and non-filtered detectors include 1990-1991 Catalog, pg. 18, Non-Invasive kVp Divider, Keithley Radiation Measurements Division, Keithley Instruments, Inc., Cleveland, Ohio.
References with respect to PC data acquisition include The Handbook of Personal Computer Data Acquisition, 7th Edition, Section 7, Data Acquisition Tutorial, Intelligent Instrumentation, Inc., Tucson Ariz. (1994).
References with respect to electron beam processors include McLaughlin, W. L., Boyd, A. W., Chadwick, K. H., McDonald, J. C., and Miller, A., Dosimetery for Radiation Processing, Taylor and Francis, (1989), 2.3 Accelerator sources, pg. 44 et seq.
References with respect to electron beam processing included the following:
"Food Irradiation in Developing Countries: A Practical Alternative", Loaharanu, Paisan, IAEA Bulletin 1, 30-35, (1994), IAEA, P. O. Box 100, A-1400, Vienna, Austria. PA1 Radiation Sterilization, Session IV, pp. 207-291, Sterilization of Medical Products, Proc. of Int'l. Kilmer conf., Vol. VI, ed. R. F. Morrissey, Brussels, Belgium, (1993); Polyscience Publications Inc., Morin Heights, Canada. PA1 Nablo, S. V., "Electron Beam Processing Machinery", Ch. 9, pp. 503-554, Radiation Curing in Polymer Science and Technology, ed. J. P. Fouassier and J. F. Rabek, Elsevier Applied Science, London (1993). PA1 Miller, A., "Status of Radiation Processing Dosimetry", Radiat. Phys. Chem. 42, 4-6, pp. 731-738, (1993).